This invention relates to a high-low speed range switching type valve mechanism for an internal combustion engine and more particularly to an improved and simplified arrangement for selectively varying the lift of a valve in response to engine conditions.
In order to further improve the performance of internal combustion engines in response to varying load and operating conditions, various devices have been provided for changing the effective lift of the engine valves. By changing the lift, the performance can be more closely tailored to the optimum for these various running and operating conditions. However, most of the mechanisms that have been proposed for this operation have a number of disadvantages due primarily to their complexity.
For example, one type of mechanism that has been proposed employs a pair of rocker arms each of which is associated with a given valve and each of which cooperates with a respective cam lobe having a different lift characteristic. Generally, one of these rocker arms directly operates the valve and the other rocker arm is selectively coupled to that one rocker arm or uncoupled from it to determine which cam lobe will control the valve lift characteristics.
Examples of this type of structure are shown in U.S. Pat. No. 4790274, issued Dec. 13, 1988 and now reissued as RE33411, Oct. 30, 1990; U.S. Pat. No. 4844023, issued Jul. 4, 1989 and U.S. Pat. No. 4869214, issued Sep. 26, 1989. As shown in these patents, each rocker arm is pivotally supported at one end by the engine body and has its other end engaged with the valve stem either directly or through the remaining rocker arm via a coupling mechanism. The cam lobes engage the respective rocker arms between their ends.
One disadvantage with this type of construction is that because the rocker is engaged between its ends, there is an amplified force exerted on the valve, which has a bending component. This requires the valve stem to be of greater strength and greater diameter. As a result, the valves are heavier. Accordingly the inertia of the engine reciprocating components increases and engine performance is deteriorated.
Another disadvantage with this type of mechanism is that the coupling mechanism between the rocker arms and its actuating device are all carried by one or both of the rocker arms. This further increases the reciprocating mass of the rocker arm resulting in deteriorated engine performance.
It is, therefore, a principal object of this invention to provide an improved variable valve lift timing mechanism wherein the engagement of the cam lobe with the rocker arm and valve is aligned so as to eliminate bending stresses.
Another object of this invention is to provide a variable valve lift mechanism of this type wherein not all of the components of the coupling arrangement between the rocker arms are carried by the rocker arms, thus reducing the weight of the rocker arms.
A first feature of the invention is adapted to be embodied in a valve operating mechanism for operating a flow controlling poppet valve of an internal combustion engine. The poppet valve has a stem portion supported for reciprocation about a valve axis in the engine for control of the flow through a port thereof by a head portion of the valve. The valve operating mechanism is comprised of a camshaft supported for rotation about a camshaft axis that is intersected by the valve axis. The camshaft has at least two cam lobes of different lift characteristics juxtaposed to the valve. A pair of rocker arms are journalled for pivotal movement about an axis offset from the camshaft axis. The rocker arms each have a follower portion engaged with a respective one of said cam lobes. One of the rocker arms has an actuating portion associated with the valve stem. An interlocking coupling selectively couples the rocker arms for simultaneous movement or permits only the one rocker arm to transmit movement to the associated valve.
Another feature of the invention is also adapted to be embodied in a valve operating mechanism for operating a flow controlling poppet valve of an internal combustion engine. In accordance with this other feature, the poppet valve has a stem portion supported for reciprocation about a valve axis in the engine for control of the flow through a port thereof by the head portion of the valve. The valve operating mechanism comprises a camshaft supported for rotation about a camshaft axis. The camshaft has at least two cam lobes of different lift characteristics associated with the valve. A pair of rocker arms are journalled for pivotal movement about an axis offset from the camshaft axis. The rocker arms each have a follower portion engaged with a respective one of the cam lobes. One of the rocker arms has an actuating portion associated with the valve stem. A coupling element is carried by one of the rocker arms and is engageable with the other of the rocker arms for selectively coupling the rocker arms for simultaneous movement or permitting only the one rocker arm to transmit movement to the associated valve. An actuating element is supported independently of the rocker arms for moving the coupling element.